The origin check is an exact match including the context path, so users had
to type "https://collab.lge.com/main" precisely. GET /api/v1/config now
returns the approved origins in canonical form (non-secret: they are the only
destinations the backend will talk to), and the UI swaps the URL text field
for a select listing them, keeping the element id, focus handling and the
Test connection flow unchanged. The text field remains the fallback when the
fetch fails. Backend validation of the submitted URL is untouched. Mock server
serves the endpoint; contract, API and e2e tests cover it.
